1. Field of the Invention
This invention relates to an adjustable proportioning valve for vehicle hydraulic brake systems and more particularly to an automatically adjustable motorized proportioning valve for such brake systems.
2. Description of the Prior Art
It is known in the art to provide a pressure proportioning valve in a vehicle brake system that includes a master cylinder and front and rear wheel brakes. A typical proportioning valve limits the pressurization of the rear brakes relative to the front brakes in response to a master cylinder pressure that exceeds a predetermined minimum. During initial buildup of master cylinder pressure, the front and rear brakes are substantially equal. When the master cylinder pressure exceeds a predetermined minimum, the proportioning valve becomes effective to limit further increases in the pressure at the rear brakes whereby the front-to-rear pressures are at a ratio of less than one-to-one. U.S. Pat. No. 3,423,936 discloses such a proportioning valve.
A pressure proportioning valve interposed in a brake system functions to compensate for differences in the effectiveness of the front and rear brakes. The differences in effectiveness may be due, in part, to differences in the types of brakes employed (for an example, disc brakes on the front wheels and self-energizing drum brakes on the rear wheels) and also, for the dynamic weight shift to the front axle during a brake application. It is the weight shift which produces the principal need for a proportioning valve.
The imbalance between the effectiveness of the front and rear brakes is particularly apparent in a truck and varies substantially depending upon the amount of load which the truck carries. In a fully loaded truck, the weight on the rear axle may be such that the rear brakes having a braking capability that is almost as great as that of the front wheels. When the same truck is in an unladen condition, however, its rear brakes will not be nearly as effective as the front brakes. In such an unladen truck, equal pressure at the front and rear brakes would produce rear wheel skid when the master cylinder pressure is high enough to produce a moderate rate of vehicle deceleration.
The proportioning valve for a passenger automobile brake system is provided with a fixed split point, that is, a fixed point in the buildup of master cylinder pressure at which the proportioning valve becomes effective to limit or retard further increases in pressure at the rear brakes. Because the amount of load carried by a passenger automobile is not substantial in relationship to the total weight of the vehicle, the relative effectiveness of the front and rear brakes is not changed substantially by vehicle load changes. Therefore, a proportioning valve having a fixed split point is acceptable in passenger cars.
In a truck, however, where the relative effectiveness of the front and rear brakes changes greatly depending upon the load of the vehicle, it is desirable to have a proportioning valve with a split point that may be modulated in response to the load on the vehicle. U.S. Pat. No. 4,062,597 issued to Sawyer et al on Dec. 13, 1977 and U.S. Pat. No. 3,614,175 issued to Lawson on Oct. 19, 1971 disclose proportioning valves that are directly connected to a spring mechanism which varies the minimum master cylinder pressure point where the valve becomes effective to modulate rear brake line pressure (i.e. split point) depending upon the loads placed upon the spring mechanism.
U.S. Pat. No. 3,257,153 issued to Strifler on June 21, 1966 also discloses a load sensitive proportioning valve which has a certain geometry to minimize the effect of dynamic horizontal forces on the proportioning valve while the vehicle is in motion.
Expensive anti-skid control circuits have also been developed to prevent wheels from skidding. One such anti-skid control device is disclosed in U.S. Pat. No. 4,050,743 issued to Knight on Sept. 27, 1977 is for a pneumatic brake system.
Another pneumatic brake system which uses electric braking blended with direct pneumatic braking is disclosed in U.S. Pat. No. 4,073,541 issued to Nagase on Feb. 14, 1978.